In.sub.0.53 Ga.sub.0.47 As/InP photodetectors are known for the 1.0-1.7 micron region of the spectrum for fiber optic and instrumental applications. These standard lattice matched photodetectors are illustrated in FIG. 1. An InP substrate wafer 12 has a buffer layer of n-InP 14 formed thereon. An active layer of n-In.sub.0.53 Ga.sub.0.47 As 16 is deposited using vapor phase epitaxy (VPE) techniques. A cap layer 18 of n-InP is deposited over the active layer 16 and a final silicon nitride passivation layer 20 is deposited thereover. Zinc is diffused through the cap layer 18 into the active layer 16 to create a p-n junction 22 therein. A bilevel Au--Zn/Ti--Pt--Au 24 contact is deposited as the anode. The InP substrate 12 is then lapped and polished and a final Au--Sn electrode 26 is deposited as the other electrode.
More recently, with the advent of optical fibers that operate at about 2.5 microns, in order to extend the spectral range of these photodetectors beyond 1.7 microns, photodetectors having graded compositional layers of In.sub.x Ga.sub.1-x As have been suggested. These arrays extend their range of spectral response for these photodetectors out to about 3 microns. By adjusting the composition of the active InGaAs layer, photodetectors having varying spectral response can be made, such as a cutoff wavelength of 1.7 microns (wherein x=0.53, E.sub.g =0.73eV); 2.2 microns (wherein x=0.71, E.sub.g =0.56eV); and 2.6 microns (wherein x=0.82, E.sub.g =0.47eV) using hydride vapor phase epitaxy (VPE) techniques. Arrays of these photodetectors having pixel sizes of 13.times.500 microns have a center-to-center spacing of 25 microns. Such arrays are useful for example, for satellite projects such as Scanning Imaging Absorption Spectrometer Atmospheric Chartography (SCIAMACHY) which monitors constituents in the upper atmosphere, including ozone, water, methane, carbon monoxide and carbon dioxide. By tailoring the various constituents of the detectors, the peak responsivity can detect various of these constituents located at different wavelengths in the light spectrum.
The biggest drawback of these current photodetector arrays is that they are lattice mismatched devices with high leakage current. Limiting the amount of leakage current in the photodetectors is important because high leakage current limits the signal to noise ratio required to detect atmospheric constituents. Optimally leakage currents on the order of 7fA@ 150K and a reverse bias of about 10 mV for a 2.6 micron cutoff wavelength array would be highly desirable. However, such levels are unknown at present.
It is known that lattice mismatch between successive, compositionally different, InGaAs layers creates dislocations in the grown layers. The dislocations are electrically active as generation-recombination centers. This lattice mismatch contributes to leakage current in InGaAs photodetectors, and a reduction in the number of dislocations will result in improved device performance.
If leakage currents of InGaAs photodetectors can be decreased, their sensitivity, yield and signal to noise ratio would be increased. Thus it would be highly desirable to be able to produce InGaAs photodetectors with lower leakage current.